1. Field of the Invention
The present invention relates to a rechargeable battery and its connection structure with a protection circuit.
2. Description of the Background
Recently, many types of rechargeable batteries that have a high power storage capacity have been developed. Examples of widely-used rechargeable batteries include nickel metal hydride (Ni-MH) batteries, lithium (Li) polymer batteries, and lithium ion (Li-ion) batteries.
A bare cell of a rechargeable battery is formed by preparing an electrode assembly with a positive electrode, a negative electrode, and a separator interposed between the electrodes. The electrode assembly is housed in a can made of aluminum or an aluminum alloy. The can is closed with a cap assembly. An electrolyte is injected into the can, and the cap assembly is sealed. Since the can comprises lightweight aluminum or an aluminum alloy, the resulting batteries may be lightweight and may be used without erosion at a high voltage for a long time. Typically, an electrode terminal is insulated from the cap assembly by a gasket that is provided to an upper portion of the bare cell. The electrode terminal is coupled with the positive electrode or the negative electrode of the electrode assembly to form a positive terminal or a negative terminal of the bare cell. The can has the opposite polarity to that of the electrode terminal.
The electrode terminal of the bare cell in the sealed rechargeable battery is coupled with terminals of safety devices such as a positive temperature coefficient (PTC) device, a thermal fuse, and a protection circuit module (PCM). These safety devices prevent destruction of the rechargeable battery by interrupting current flow in the event of a thermal runaway or an abnormal increase in voltage.
Typically, a conductor structure called a connection lead connects the safety device to the positive electrode or the negative electrode of the bare cell of the rechargeable battery. The connection lead may comprise nickel, a nickel alloy, or nickel-plated stainless steel.
The bare cell and the safety device that is coupled with the bare cell are enclosed by a hard case to form a hard battery pack.
Since the connection lead is made of nickel or the like, the connection lead may cause problems when it is welded to a lower surface of the can that is made of an aluminum alloy. More specifically, since nickel is infusible at a low temperature and is highly conductive, it is hard to weld nickel to aluminum using an ultrasonic or resistance welding process. Therefore, a laser welding process in which a laser beam is illuminated at contact point between the can and the connection lead is used so that the contact point can be partially fused. However, during laser welding, the illuminated laser beam and the associated electrical charging phenomenon may have an electrical impact on the protection circuit that is connected to an end portion of the connection lead. This may destroy the safety device deteriorate its performance, in turn harming the reliability of the rechargeable batteries.
To overcome the problem associated with the laser welding process, U.S. Pat. No. 5,976,729 discloses an approach in which a lead plate made of nickel is welded to a lower surface of a can made of nickel by laser welding. According to the approach, the lead plate that is coupled with the protection circuit is resistance-welded to the lower surface of the can.
FIG. 1 is an exploded perspective view of a conventional can-type rechargeable battery comprising a bare cell 20, a can 11, and a protection device 40 which include protection circuit 42 and PTC 41 (Positive Thermal Coefficient 41). The reference number 21 is an electrode terminal, 23 is an electrolyte injecting hole, and 44 and 46 are connection leads. FIG. 2 is a bottom view of a lead plate 25 welded to a connection portion of a lower portion 20 of the can 11 of the conventional can-type rechargeable battery.
A demand for a compact, lightweight, high-capacity battery requires, cans (containers) that have a large internal volume and a small size and thickness. Therefore, it is difficult to properly attach the lead plate 25 to the lower portion 20 of the can 11 by laser welding. More specifically, since the can is very thin, the welding intensity must be adjusted so that leakage of electrolyte from a laser-welded portion 27 can be prevented.
In addition, if there is a gap between the lead plate 25 and the lower portion 20 of the can 11 welded thereto, a hole may be generated on the lead plate 25 when the connection lead 46 is welded. If the lead plate is not welded to a predetermined portion, the connection lead 46 may be directly welded to the lower portion 20 of the can, so that the aforementioned conventional problems cannot be overcome.
In addition, if a safety vent 29 is provided to the lower portion 20 of the can, the safety vent 29 may suffer thermal impact during the laser welding process, causing damage to the seal of the bare cell. These problems eventually result in a lower production yield.